The present invention relates to a transmission method and transmission apparatus in an OFDM-CDMA communication system, and more particularly to a transmission method and transmission apparatus in an OFDM-CDMA communication system that divides subcarriers into groups of subcarriers having close or equivalent propagation environments (for example electrical power), and transmits subcarrier components that have been multiplied by channelization code by subcarriers of the same group.
Much attention has been paid to multicarrier communication methods as a next-generation mobile communication method. By using a multicarrier communication method, wideband, high-speed transmission is possible, and by making the bandwidth of each subcarrier narrow, it is possible to reduce the effect of frequency selective fading. Particularly, by using an Orthogonal Frequency Division Multiplexing (OFDM) method, it is possible to further improve the frequency utilization efficiency, and by using a guard interval after each OFDM symbol, it is possible to eliminate the effect of intersymbol interference.
Also, recently, research concerning a multicarrier CDMA method (OFDM-CDMA) is actively being pursued, and application to a next-generation wideband mobile communication method is being studied. In OFDM-CDMA transmission, each symbol is spread (for example, multiplied by spreading code (channelization code) of a length N that corresponds to the spreading factor) to create a plurality of subcarrier components, and those subcarrier components are respectively transmitted by corresponding subcarriers. By performing spreading in the frequency direction in this way, subcarriers, whose frequency spacing has been separated receive independent fading by frequency selective fading.
FIG. 12 shows an example of the construction of a transmission apparatus (base station) for an OFDM-CDMA communication system. Here, a data modulation unit 11 modulates the user's transmission data, and converts the in-phase component and orthogonal component to complex baseband signals (symbols). A time multiplexing unit 12 performs time multiplexing of pilots (common pilots) of a plurality of symbols in front of the transmission data. A serial-to-parallel conversion unit 13 converts input data to parallel data for M number of symbols, divides each symbol into N number of branches, and inputs the result to a spreading unit 14. The spreading unit 14 comprises M number of multiplication units 141 to 14M, and each of the multiplication units 141 to 14M multiplies the branch symbols by respective channelization code and output the results. In other words, the spreading unit 14 performs spreading by multiplying one branched symbol by channelization code (C1 to CN) having length N, and outputs signals S1 to SN that are spread by the chips (C1, . . . , CN) of the channelization code. As a result, subcarrier signals S1 to SMN for multicarrier transmission by M×N number of subcarriers are output from the spreading unit 14. That is, the spreading unit 14 performs spreading in the frequency direction by multiplying each of M number of symbols by channelization code made up of chips C1 to CN. The channelization code that is used in spreading differs for each user or control channel, and code that is orthogonal with each other is used as the channelization code for each user or control.
A code multiplexing unit 15 multiplexes the subcarrier signals that were generated as described above with subcarrier signals of other users and subcarrier signals for control that were generated using a similar method. In other words, for each subcarrier, adding units 151 to 15MN of the code multiplexing unit 15 combine and output the subcarrier signals for a plurality of users and subcarrier signals for control that correspond to the subcarrier. An IFFT (Inverse Fast Fourier Transform) unit 16 performs IFFT (inverse Fourier transformation) of the subcarrier signals that are input in parallel, and converts them to M×N number of subcarrier signal components (OFDM signal) on the time axis. A guard interval insertion unit 17 inserts guard intervals having a specified length into the ODM signal, an orthogonal modulation unit 18 performs orthogonal modulation of the OFDM signal in which guard intervals have been inserted, and a radio transmission unit 19 performs UP conversion of the frequency to a radio frequency, as well as performs high-frequency amplification and transmits the signal from an antenna.
The total number of subcarriers is (the number of parallel lines M)×(spreading factor N). Also, in the propagation path, in order to receive different fading for each subcarrier, pilots are time multiplexed on all subcarriers, and on the receiving side, fading compensation (channel estimation/channel compensation) is performed for each subcarrier.
FIG. 13 shows an example of construction of a receiving apparatus in an OFDM-CDMA communication system. A radio receiving unit 21 performs frequency conversion of a received multicarrier signal, and an orthogonal demodulation unit 22 performs orthogonal demodulation of the received signal. A timing synchronization and guard interval removal unit 23 performs timing synchronization of the received signal, removes the guard intervals from the received signal, and inputs the result to a FFT (Fast Fourier Transform) unit 24. The FFT unit 24 performs FFT processing using FFT window timing, and converts the time-domain signal to M×N number of subcarrier signals (subcarrier symbols).
A channel estimation unit 25 uses the pilots that were time multiplexed by the transmitting side and performs channel estimation for each subcarrier, then finds a channel compensation value for each subcarrier and inputs the values to a channel compensation unit 26, after which the channel compensation unit 26 multiplies each subcarrier signal by a channel compensation value to perform fading compensation (channel compensation). In other words, the channel estimation unit 25 uses the pilot signal to estimate the amplitude due to fading and phase effect Ai·exp(jφi) for each subcarrier, and the multiplication units 26i (i=1 to M×N) of the channel compensation unit 26 multiply the subcarrier signals of the transmission symbols by (1/Ai)·exp(−jφi) to compensate for fading.
An despreading unit 27 comprises M number of multiplication units 271 to 27M, and the multiplication unit 271 multiplies N number of subcarrier components individually by C1, C2, . . . CN of the channelization code that is assigned for a user, and outputs the result, while each of the other multiplication units perform similar processing. As a result, despreading is performed on fading-compensated signals by using channelization code (spreading code) that is assigned for each user, and by this despreading, the signal for a desired user is extracted from the code multiplexed signal.
Combination units 281 to 28M add the N number of multiplication results that are output from the multiplication units 271 to 27M to create parallel data made up of M number of symbols, a parallel-to-serial conversion unit 29 converts that parallel data to serial data, and a data demodulation unit 30 demodulates the transmission data.
When spreading is performed in the frequency direction in OFDM-CDMA communication as described above, subcarrier components having the same numbers as shown in FIG. 14 are spread as the subcarrier components for one symbol and transmitted. FIG. 14 shows the case in which OFDM-CDMA communication is performed at specified timing ti for three symbols (M=3) at a time, where the spreading factor is taken to be four (N=4). In the figure, portions having the same number are chip data for one symbol, and in this case, during transmission, each symbol is multiplied and spread by channelization code (spreading code) having a symbol length of four. On the receiving side, despreading is performed at each timing, and one symbol comprising the four chips having the same number as in FIG. 14 is demodulated. In the OFDM-CDMA communication method, by multiplying and spreading data for each user by different channelization code at the same time ti, it is possible to perform multiplexed data communication. In this case, the channelization code for each user and for control data is orthogonal to each other, so there is no interference between each other. However, in the CDMA communication method in which spreading is performed on the transmitting side and demodulation is performed by despreading on the receiving side, it is presumed that on the receiving side the amplitude of each chip is nearly the same.
OFDM-CDMA communication differs from W-CDMA communication or the like in which spreading is performed in the time direction, in that frequency selective fading occurs due to multipaths. There are cases in which, even though the amplitude of each chip is the same in the time direction, the amplitude may greatly vary in the frequency direction. When the amplitude of each chip is not the same, othogonality of the spreading code is lost and components of other code are mixed in, so the reception characteristic becomes poor. An example of Downlink communication is explained below, however, the same method can be applied to Uplink communication.
In the OFDM-CDMA communication method, frequency selective fading occurs. As shown in FIG. 15, even when the same electrical power from the base station is allotted for each subcarrier (frequency), when a terminal (mobile station) receives the subcarriers, the power for each subcarrier differs due to the effect of the propagation path (channel). This is because signals that are transmitted from the base station arrive at the mobile station at a plurality of different times in multipaths due to reflection by buildings and the like.
Moreover, the OFDM-CDMA communication method is characterized by multiplexing a plurality of data at the same time by using orthogonal channelization code. The basis of this orthogonality is that the amplitude of each chip is the same, and when the amplitude is different, orthogonality is lost, multiplexed data interfere with each other, and the reception characteristic becomes poor. For example, in a case where a spreading factor is 4, the code 1, 1, −1, −1 and code 1, 1, 1, 1 are orthogonal. Generally in the case of codes a1, a2, a3, a4 and b1, b2, b3, b4, The orthrogonality can be determined by whether or not the condition a1×b1+a2×b2+a3×b3+a4×b4=0 is satisfied. However, when the amplitude of the first chip is twice the amplitude of the others, then 2×2+1×1+(−1)×1+(−1)×1=3, and the codes are no longer orthogonal. The portion that is not orthogonal becomes the interference component.
As shown in FIG. 14, in a normal OFDM-CDMA communication method in which spreading is performed in the frequency direction, the N number of subcarrier components resulting from multiplying data for one symbol by spreading code having a spreading factor N (=4) are allotted in order of frequency to each subcarrier. The reason for this allocation is that there is a high possibility that there is very little difference in the amplitudes of chips having frequencies that are close. However, as shown in FIG. 15, in an environment where there is a large fluctuation in frequency selective fading, the amplitudes may be greatly different even though the frequencies are close. In that case, the data of the spreading code for each terminal is no longer orthogonal and interference occurs, and thus demodulation cannot be performed properly. As a method for preventing this deterioration, proposed is prior art (see JP 2001-86093A) in which the amplitudes of each of the chips are made the same and a loss of orthogonality is avoided by increasing the weighting on the receiving side for subcarriers having small amplitudes. By doing this, it is possible to suppress interference with other code signals, however, it is impossible to execute maximum ratio combining. For the subcarriers that having small amplitudes, it is preferable to combine them as they are, thereby the maximum ratio combining becomes possible, and communication quality becomes high. The prior art performs the opposite of maximum ratio combining, so there is little improvement in quality.